1. Field of the Invention
The present invention relates to switching circuits suitable for use in the deflection circuitry associated with cathode ray tubes. More particularly, the present invention relates to such circuits which include two semiconductor switching elements connected in series for switching large voltages.
2. Description of the Prior Art
Electron beams generated in cathode ray tubes are controlled by magnetic fields within the tube which deflect the beam. Typically, coils are employed to create the magnetic fields.
In many cathode ray tube applications, including televisions and oscilloscopes, the electron beam sweeps across the screen at a constant speed. This is accomplished by passing a linearly increasing current through one of the deflection coils. A simplified circuit for accomplishing this result is illustrated in FIG. 1. Deflection coil 10 is connected in a circuit with capacitor 12 and voltage source 14. Switch 16 is connected in parallel with capacitor 12.
FIG. 2 illustrates the waveform of the current passing through coil 10, and FIG. 3 illustrates the voltage across capacitor 12. If voltage source 14 is energized with switch 16 closed, current will flow through coil 10 in the direction of arrow 18 and will increase as shown in FIG. 2 between times a and b. At some appropriate point (time b), switch 16 is opened which causes current to flow through capacitor 12 instead of switch 16, thus charging capacitor 12. As soon as current begins to flow through capacitor 12, the magnitude of the current decreases (from time b to time c), charging capacitor 12. Then, the energy stored in capacitor 12 causes current to begin flowing in a direction opposite to arrow 18 (from time c to time d). This current reaches a peak in the opposite direction at time d when the energy in capacitor 12 is completely exhausted. At some point, switch 16 is closed and the cyle repeats itself.
As the current through coil 10 ramps up between times d and e, the electron beam sweeps across the screen. As the current ramps down, between times b and d and times e and f, the beam retraces back to its original position.
When switch 16 is opened while the current flowing through coil 10 is at a maximum, a large voltage develops across capacitor 12 as illustrated in FIG. 3. Of course, in practice, switch 16 is not mechanical but a semiconductor switching device. A problem arises in that the maximum voltage that is applied across capacitor 12, and thus switch 16, may exceed 1,000 volts. In their present stage of development, this is an unusually high voltage for semiconductor switching elements to handle.
To relieve this problem, two semiconductor switching elements have been connected in series. Thus, each semiconductor switching element need only handle approximately 500 volts, greatly reducing the expense and increasing the availability of the switching element.
A problem arises, however, in biasing the two semiconductor switching elements. Typically, the voltage difference between the control terminal and an appropriate one of the power terminals of the semiconductor switching element determines whether the element is conductive or not. When the switching elements are rendered nonconductive, the appropriate power terminal of one of the switching elements will rise up to 500 volts while the appropriate power terminal of the other switching element will remain close to zero volts. If a zero volt control signal is applied to both switching elements to render them nonconductive, the potential across the control terminal and the appropriate power terminal of one of the switching elements may reach or exceed 500 volts, which may destroy the switching element.
To overcome this problem, several approaches have been employed in the past. For example, transformers have been utilized to provide common mode isolation to prevent excessive voltages from developing across the terminals of a switching element. Thus, the primary of the transformer receives the control signal and the secondary of the transformer is connected across the control terminal and the appropriate power terminal of the switching element. When the voltage on the power terminal rises, the voltage on the control terminal also rises to the same level plus the voltage generated by the secondary.
However, problems exist with the use of a transformer or any other magnetic circuitry. Magnetic circuitry does not function well at high frequencies. For some cathode ray tube applications, the frequency of the control signal may be so high that transformers may not function efficiently. Furthermore, transformers tend to be bulky, heat generating devices which may not be suitable for some applications.
Another approach has employed optical drivers to provide a high level of isolation. Although an optical system can provide the isolation, at the present state of development, optical drivers are slow, operating in the microsecond range. In some cathode ray tube applications, the entire horizontal sweep must be accomplished in 12 microseconds and the retrace must occur in 3.5 microseconds. Therefore, the semiconductor switching elements must be switched on the order of 50 to 100 nanoseconds. Optical drivers simply cannot respond at this speed.